Traditionally in heading reference systems used for providing a heading relative to magnetic north, such as Attitude and Heading Reference Systems, Air Data and Attitude Heading Reference Systems, Inertial Navigation Systems, or Integrated Standby Systems of the type conventionally employed in aircraft, the heading gyro is periodically corrected by means of data from an external magnetometer that relies on the earth's magnetic field to provide a heading relative to magnetic north. However, the accuracy of such readings can significantly be affected by the presence of local soft iron, such as from local electric circuits on board the aircraft, resulting in meaningful errors in the calculation and ultimate display of the magnetic heading relative to true magnetic north. This can result in serious problems, such as with respect to the proper guidance of the aircraft along its designated flight path. Because of the seriousness of this problem, as well as the desire for a cost effective and efficient solution various attempts have been made to try and avoid these effects which, while helpful, have not totally achieved the objective. One such prior art attempt to try and avoid the effects of the local presence of soft iron which provides these undesirable magnetic disturbances, has involved carefully locating the external magnetometers in aircraft areas where there are minimal local magnetic disturbances present; however, this approach has not proved to be sufficiently cost effective and efficient since it involves locating the external magnetometers on the aircraft wings away from the other electrical devices which can cause such soft iron magnetic disturbances. Such an approach introduces a significant cost due to the presence of such factors as increased installation costs as well as the increased cost of additional units.
In addition, although the use of multiple reference systems installed at different locations of a vehicle can alleviate some of the soft iron effects by providing redundancy through the ability to obtain supplemental heading measurements, it remains a highly costly solution in terms of installation and maintenance. Furthermore, such multiple reference systems commonly operate independently and typically disable the unit that is exhibiting erroneous measurements due to magnetic disturbances instead of compensating by taking into account the measurements of the remaining systems.
This problem of soft iron magnetic disturbances affecting the provision of a correct heading relative to magnetic north while significant with respect to aircraft guidance is not limited to aircraft instruments and is important as well in the guidance of other vehicles which utilize heading reference systems for guidance. Thus, the same problem can occur in any vehicle and any inertial system that normally requires a source of heading that can be subject to variations as a result of local electric/magnetic field changes. Consequently, it is believed that the present invention is applicable to any magnetic heading indication system where disturbances of local soft iron can result in errors in magnetic heading determinations.
Prior art attempts at solving such problems, in applicant's view, are not as efficient or cost effective as the claimed invention and, thus, have not adequately solved the problem. Examples of other such prior art attempts, which applicant believes do not satisfactorily solve the problem, are described, by way of example, in U.S. Pat. No. 4,414,753, entitled “Process for Compensating the Magnetic Disturbances In The Determination of a Magnetic Heading and Devices for Carrying Out This Process”; U.S. Pat. No. 5,737,226, entitled “Vehicle Compass System With Automatic Calibration”; and U.S. Pat. No. 5,878,370, entitled “Vehicle Compass System With Variable Resolution”. Other prior art attempts at compensating for errors in the measurement of magnetometers, which applicant also believes do not satisfactorily solve the problem presented here, are described, by way of example in U.S. Pat. No. 5,682,335, entitled “Method and Device for Simultaneous Identification and Correction of Errors in the Measurements of a Magnetometer”; U.S. Pat. No. 7,146,740, entitled “Methods and Apparatus for Automatic Magnetic Compensation”; U.S. Pat. No. 6,860,023, entitled “Methods and Apparatus for Automatic Magnetic Compensation”; U.S. Pat. No. 5,990,679, entitled “Method Using Corrective Factors for Determining a Magnetic Gradient”; U.S. Pat. No. 5,321,631, entitled “Method and Self-Contained System for Harmonizing Equipments On Board a Vehicle Using Means of Measuring the Earth's Gravitational and Magnetic Fields”; U.S. Pat. No. 4,843,865, entitled “Method of Calibrating a Compass Heading”; U.S. Pat. No. 4,733,179, entitled “Method of Determining an Interference-Magnetic Field in a Motor Vehicle Provided with an Electronic Compass”; and U.S. Pat. No. 4,005,358, entitled “Magnetometer with Out-of-Phase Correction”.
Thus, it is believed that there is a need for an efficient and cost effective solution for avoiding the effects of local soft iron on magnetic heading calculations in magnetic heading reference systems whether in an aircraft or any other vehicle employing such a system for proper guidance.